Contact sensitive device

ABSTRACT

A contact sensitive device includes a member capable of supporting bending waves and a plurality of sensors mounted on the member for measuring bending wave vibration in the member. The sensors measure the bending wave signals and by calculating a phase angle for each measured bending wave signal and a phase difference between the phase angles of least two pairs of sensors so that at least two phase differences are calculated, the location of the contact can be determined.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit under 35 U.S.C. § 119(e) ofapplication No. 60/432,024, filed Dec. 10, 2002, which application isincorporated herein by reference.

BACKGROUND

1. Field of the Invention

The invention relates to contact sensitive devices.

2. Description of the Related Art

Visual displays often include some form of touch sensitive screen. Thisis becoming more common with the emergence of the next generation ofportable multimedia devices such as palm top computers. The mostestablished technology using waves to detect contact is Surface AcousticWave (SAW), which generates high frequency waves on the surface of aglass screen, and their attenuation by the contact of a finger is usedto detect the touch location. This technique is “time-of-flight”, wherethe time for the disturbance to reach one or more sensors is used todetect the location. Such an approach is possible when the mediumbehaves in a non-dispersive manner i.e. the velocity of the waves doesnot vary significantly over the frequency range of interest.

SUMMARY

According to one embodiment of the invention, there is provided acontact sensitive device comprising a member capable of supportingbending waves, a plurality (e.g. three or more) sensors mounted on themember for measuring bending wave vibration in the member, whereby eachsensor determines a measured bending wave signal. A processor calculatesa location of a contact on the member from the measured bending wavesignals, in that the processor calculates a phase angle for eachmeasured bending wave signal, then calculates a phase difference betweenthe phase angles of least two pairs of sensors from which the locationof the contact is determined.

According to another embodiment of the invention, there is provided amethod of determining information relating to a contact on a contactsensitive device that includes providing a member capable of supportingbending waves and a plurality of sensors (e.g., three or more) mountedon the member for measuring bending wave vibration in the member,applying a contact to the member at a location, using each sensor todetermine a measured bending wave signal and calculating the location ofa contact from the measured bending wave signal by calculating a phaseangle for each measured bending wave signal, calculating a phasedifference between the phase angles of at least two pairs of sensors anddetermining the location of the contact from the at least two calculatedphase differences.

The following features may be applied to both the device and the methodwith the processor being adapted to provide many of the calculations orprocessing steps of the method.

Reflected waves may be suppressed by placing an absorber in contact withthe edges of the member. The mechanical impedance of the absorber andmember may be selected so as to significantly reduce or minimizereflections of bending waves from the edges of the member. Inparticular, the impedances may be selected such that bending wave energyis strongly absorbed in a frequency band around a chosen frequency ω₀.The impedance of the absorber may be selected to be both resistive andcompliant. The impedances may be selected to satisfy the followingequation:Z _(T)=−iZ_(B)(ω₀)where Z_(T) is the termination impedance of the absorber, Z_(B) is themechanical impedance of the edge of the member, and i is the imaginarynumber=√{square root over (−1)}.

The absorber may be made from foamed plastics which may have open orclosed cells and may be polyurethane or polyvinylchloride. For examplethe foam may be a soft PVC predominantly closed cell foam such as MIERS™or a medium to high density, open cell polyurethane foam. The absorbermay extend substantially around the periphery of the member. Theabsorber may act as a mounting which supports the member in a frame orto another surface.

The member may comprise a raised pattern on its surface whereby acontact drawn across the surface provides a variable force to the memberto generate bending waves in the member. The pattern may be periodic, orquasi-periodic with a statistically well-defined spatial distribution ofundulations. The pattern may be random whereby a contact travelling overthe surface of the member generates a random bending wave signal. Therandom relief pattern may be an anti-reflective coating, an anti-glaresurface finish, or an etched finish such as those which are found onmany known transparent panels placed in front of electronic displays.

The member may be a liquid crystal display screen comprising liquidcrystals utilized to excite or sense bending wave vibration in themember.

Each measured bending wave signal may be processed by a band-pass filterwith a pass-band centered at the chosen frequency ω₀ and having abandwidth of Δω. The bandwidth Δω of the filter is preferably chosen toaddress the Doppler effect whereby a bending wave arrives at a pointwith a different frequency from its original frequency. Accordingly, thebandwidth preferably obeys the relationship:Δω>>2k(ω₀)ν_(max)where ν_(max) is the maximum lateral velocity of the contact across thesurface, e.g. if the contact is provided by a stylus, ν_(max) is themaximum velocity that a user is capable of moving the stylus.

The phase of each filtered signal may be measured by comparison with areference signal. The reference signal may have a frequency ω₀. Themeasured phase is the average phase difference between the input andreference signals, optimally measured over the interval 2π/Δω.Alternatively, the reference signal may be derived from a filteredsignal from a second sensor in which case, the measured phase is thephase difference between two input signals.

The phase differences may be calculated at intervals of 2π/Δω which maybe intervals of less than 10 ms. The reference and input signals may befed to a phase detector. Output from the phase detectors may be fedthrough low-pass filters having frequency cut-offs of approximatelyΔω/2, then through digitisers and finally through a processor tocalculate the phase angle θ.

The instantaneous phases, θ_(l)(t) and θ_(m)(t), of two measured bendingwave signals may satisfy the phase difference equation:Δθ_(lm)=θ_(l)−θ_(m) =k(ω₀)Δx _(lm)+2πn _(lm)where Δx_(lm)=x_(l)−x_(m) (x_(m) and x_(l) being the distance from thecontact location to each sensor labelled m and l respectively), and k(ω)is the wavevector. This equation may be satisfied if the path lengthdifference between two sensors is less than the coherence length of thebandpass filter, which is defined as$x_{c} = \frac{2\quad{\pi\omega}_{0}}{\Delta\quad\omega\quad{k\left( \omega_{0} \right)}}$The coherence condition is therefore |Δx_(lm)|<<x_(c). If the coherencecondition is not satisfied, the above phase equation may not besatisfied.

Thus, values of n_(lm) and the phase angle difference are required todetermine the location of the contact. The shape of the member may beselected to constrain the magnitude of Δx_(lm) to values less than halfof one wavelength, ie. |Δx_(lm)|<π/k(ω₀). In this case, where allpossible values of Δx_(lm) satisfy the condition |Δx_(lm)|<π/k(ω₀),there is only one value of n_(lm) which is the integer n_(lm) satisfying|Δθ_(lm−)2πn_(lm)|<π. Alternatively, n may be estimated or inferred insome way.

Another class of foams that has been found to be suitable are acrylicclosed cell foams. These may have a high degree of damping andrelatively high stiffness. Such properties are particularly suited toedge termination of stiff, heavy materials such as glass. Examplesinclude 3M serial numbers 4956, 4910, 4950, and 4655.

Each phase angle difference in combination with the range of possiblevalues of the integer n_(lm) may be used to generate a series of pathlength differences thereby defining a series of discrete hyperboliccurves on the surface of the member, denoting possible locations of thecontact. The location of the contact may be determined by plotting eachhyperbola defined by each path length difference and selecting a pointat which a large number of the hyperbolae intersect or nearly intersect.This point is likely to be the true location of the contact.

Where n_(lm) is unknown, the minimum number of series of hyperbolaerequired to determine the contact location is three and the likelihoodof determining the correct location of the contact is increased byincreasing the number of hyperbolae to be plotted. Multiple sensors maybe used whereby a phase angle difference may be calculated for each pairof sensors thus generating multiple hyperbolae. In this embodiment, theminimum number of sensors is three.

Alternatively, where n_(lm) is unknown, the measured bending wave signalfrom each sensor may be divided into two or more discrete frequencybands whereby a phase angle difference may be calculated for eachfrequency band and for each pair of sensors. Although multiple phaseangle differences may be calculated from a single pair of sensors, thephase angle differences at different frequencies are derived from thesame path length difference. Thus the minimum number of sensors isthree. The dividing of the frequency bands may be achieved by processingthe bending wave signals by at least two band-pass filters havingdifferent pass-band frequencies. For example, using two band-passfilters having frequencies ω₀+ω_(δ) and ω₀−ω_(δ) the phase angledifferences Δθ_(a), Δθ_(b) from two sensors may be defined asΔθ_(a) =k(ω₀+ω_(δ))Δx+2πn _(a)Δθ_(b) =k(ω₀−ω_(δ))Δx+2πn _(b)where Δx is a single path-length difference defined by the contact andthe position of the sensors.

Therefore the values of n_(a)and n_(b)may be selected so that themeasured phase angle differences infer similar values of the path-lengthdifference. There may be only one combination of values (n_(a), n_(b))for which this is possible. In this case the true value of thepath-length difference may be determined. The correct combination(n_(a), n_(b)) may be determined as the combination of values thatminimise the expression:${\frac{{\Delta\quad\theta_{a}} - {2\pi\quad n_{a}}}{k\left( {\omega_{0} + \omega_{\delta}} \right)} - \frac{{\Delta\quad\theta_{b}} - {2\pi\quad n_{b}}}{k\left( {\omega_{0} - \omega_{\delta}} \right)}}$The path length difference may then be estimated as:${\Delta\quad x} = {\frac{1}{2}\left( {\frac{{\Delta\quad\theta_{a}} - {2\pi\quad n_{a}}}{k\left( {\omega_{0} + \omega_{\delta}} \right)} + \frac{{\Delta\quad\theta_{b}} - {2\pi\quad n_{b}}}{k\left( {\omega_{0} - \omega_{\delta}} \right)}} \right)}$

Where this process is repeated with two pairs of sensors, two pathlength differences may be determined, which in turn may be used todetermine the location of the contact.

Alternatively, where n_(lm) is unknown, an initial determination of thelocation of the contact may be made using the methods taught inWO01/48684 and PCT/GB2002/003073 (as summarized in FIG. 11). Thereafter,if the condition Δω>>2k(ω₀)ν_(max) holds, the phase angle differenceschange by small increments over the timescale Δt=2π/ω. Accordingly, eachvalue of n may be chosen to minimize the change in path lengthdifference.

Measured phase angle differences may contain random errors which mayresult in the selection of incorrect values of n. This error may bealleviated by evaluating the likelihood of successive sequences of n,for example by a state-space estimator such as the well known Kalmanfilter. The sequence having the maximum measure of likelihood isselected.

The state-space estimator provides an estimate of the internal state ofa system of which noisy measurements are made. A necessary input to thestate-space estimator is a statistical description of the evolution ofthe system state. An example of such a state is the set of coordinatesthat describes the position and velocity of an object in contact withthe member. It is widely known that the Kalman filter and otherstate-space estimators may provide a measure of likelihood that asequence of observed, noisy measurements are consistent with the modelof the system state.

A state-space estimator may therefore be employed to take a sequence ofa pair of path-length differences (say Δx₁₂ and Δx₃₄) taken at differenttimes (say t₁, t₂, t₃, . . . ), to estimate the system-state, i.e. theposition and velocity of the contact, at those times. Moreover, theoverall likelihood of those values of path-length difference beingconsistent with the model of the system may be evaluated.

Where the sequence of path-length differences are obtained from asequence of phase-angle differences and a set of integers (n=n(t₁),n(t₂), n(t₃), . . . ), the measure of likelihood generated by thestate-space estimator may be used to infer the likelihood that thecorrect values of n were chosen. It follows that a method for choosingthe correct sequence of integers, n, is to find the sequence for whichthe state-space estimator gives the maximum measure of likelihood.

As mentioned above, the state space estimator uses some statisticaldescription of the evolution of the system state. A suitable model forthe motion of the contact may be a simple random walk. Alternatively,the model may employ a detailed statistical description of how the usermoves the stylus or finger. One example is a statistical description ofhow the user moves a pen while writing text or individual characters.

The processor may further be adapted to include in the determinationprocedure any available information about where the contact can beexpected. For example, if the member is an input device for a graphicaluser interface where the user is presented with a choice of ‘buttons’ topress, it may be useful to assume that any contact on the member occurswithin the discrete areas corresponding to the buttons.

Alternatively, a map of the probability at which a contact is likely tooccur and which is based on the expected behaviour of the user may beused. The device may comprise a software application with a graphicaluser interface (GUI) which interacts with the operating system by meansof an application program interface (API) in which the API is adapted togenerate the probability map. The probability map may be based on thelocation, size, and frequency of use of objects presented by thegraphical user interface. The probability map may also be based oninformation about the relative likelihood of the various GUI elementsbeing activated.

The following characteristics may apply to all embodiments of theinvention. The device may comprise means for recording measured bendingwave signals from the or each sensor over time as the contact movesacross the member. The information relating to the contact may becalculated in a central processor. The sensors may be mounted at orspaced from an edge of the member. The sensors may be in the form ofsensing transducers which may convert bending wave vibration into ananalogue input signal.

The member may be in the form of a plate or panel. The member may betransparent or alternatively non-transparent, for example having aprinted pattern. The member may have uniform thickness. Alternatively,the member may have a more complex shape, for example a curved surfaceand/or variable thickness.

The device may be a purely passive sensor with the bending wavevibration and hence the measured bending wave signals being generated byan initial impact or by frictional movement of the contact. The contactmay be in the form of a touch from a finger or from a stylus which maybe in the form of a hand-held pen. The movement of a stylus on themember may generate a continuous signal which is affected by thelocation, pressure and speed of the stylus on the member. The stylus mayhave a flexible tip, e.g. of rubber, which generates bending waves inthe member by applying a variable force thereto. The variable force maybe provided by tip which alternatively adheres to or slips across asurface of the member. As the tip moves across of the member a tensileforce may be created which at a certain threshold, causes any adhesionbetween the tip and the member to break, thus allowing the tip to slipacross the surface. The bending waves may have frequency components inthe ultrasonic region (>20 kHz).

The member may also be an acoustic radiator and an emitting transducermay be mounted to the member to excite bending wave vibration in themember to generate an acoustic output. The frequency band of the audiosignal of the transducer preferably differs from and does not overlapthe frequency band of the measurements from the sensors. The audiosignal may thus be filtered, for example, the audio band may be limitedto frequencies below 20 kHz, and the vibration measurements may belimited to frequencies above 20 kHz. A sensor may have dualfunctionality and act as the emitting transducer.

The or each emitting transducer or sensor may be a bender transducerwhich is bonded directly to the member, for example a piezoelectrictransducer. Alternatively, the or each emitting transducer or sensor maybe an inertial transducer which is coupled to the member at a singlepoint. The inertial transducer may be either electrodynamic orpiezoelectric.

A contact sensitive device according to the invention may be included ina mobile phone, a laptop or a personal data assistant. For example, thekeypad conventionally fitted to a mobile phone may be replaced by acontinuous moulding which is touch sensitive according to the presentinvention. In a laptop, the touchpad which functions as a mousecontroller may be replaced by a continuous moulding which is a contactsensitive device according to the invention. Alternatively, the contactsensitive device may be a display screen, e.g. a liquid crystal displayscreen comprising liquid crystals which may be used to excite or sensebending waves. The display screen may present information relating tothe contact.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings in which:

FIG. 1 is a schematic plan view of a touch sensitive device according toone embodiment of the invention;

FIG. 2 is a schematic perspective view of the device of FIG. 1;

FIG. 3 is a schematic side view of a one-dimensional beam;

FIG. 4 a is a graph showing the amplitude of the reflection coefficientagainst frequency (Hz), the amplitude is unitless since it is a ratio;

FIG. 4 b is a graph showing the phase (in radians) of the reflectioncoefficient against frequency (Hz);

FIGS. 5 a and 5 b are schematic perspective views of alternative touchsensitive devices;

FIG. 6 is a flowchart of a method of finding the location of a contactaccording to an embodiment of the invention;

FIG. 7 a is a schematic block diagram of apparatus used for calculatingphase angles;

FIG. 7 b is a schematic block diagram of apparatus used with that ofFIG. 7 a;

FIGS. 8 a to 8 d are plan views of apparatus according to an embodimentof the invention showing the hyperbolae of path length differences;

FIG. 9 is a schematic block diagram of alternative apparatus used forcalculating phase angles;

FIG. 10 is a flow chart showing an alternative method of calculating thelocation of the contact;

FIG. 11 is a flow chart showing a method of calculating the location ofthe contact using the dispersion corrected correlation function;

FIG. 11 a is a graph of dispersion corrected correlation functionagainst time;

FIG. 12 a is a schematic block diagram of a contact sensitive devicewhich also operates as a loudspeaker, and

FIG. 12 b is a method of separating audio signal and measured bendingwave signal in the device of FIG. 12 a.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 shows a contact sensitive device 10 comprising a transparenttouch sensitive plate 12 mounted in front of a display device 14. Thedisplay device 14 may be in the form of a television, a computer screenor other visual display device. A stylus 18 in the form of a pen is usedfor writing text 20 or other matter on the touch sensitive plate 12.

The transparent touch sensitive plate 12 is a member, e.g. an acousticdevice, capable of supporting bending wave vibration. As shown in FIG.2, four sensors 16 for measuring bending wave vibration in the plate 12are mounted on the underside thereof. The sensors 16 are in the form ofpiezoelectric vibration sensors and are mounted one at each corner ofthe plate 12. At least one of the sensors 16 may also act as an emittingtransducer for exciting bending wave vibration in the plate. In thisway, the device may act as a combined loudspeaker and contact sensitivedevice.

In the following applications, U.S. patent application Ser. No.09/746,405, filed Dec. 26, 2000 entitled “Contact Sensitive Device”naming Nicholas P. R. Hill as an inventor, International PublicationNumber WO 01/48684 (International Application Number PCT/GB00/04851) andInternational Application PCT/GB2002/003073, filed Jul. 3, 2002, whichapplications are incorporated herein by reference, contact sensitivedevices and methods of using the same are described. The applicationsdescribe a device that includes a member capable of supporting bendingwave vibration and a sensor mounted on the member for measuring bendingwave vibration in the member and for transmitting a signal to aprocessor whereby information relating to a contact made on a surface ofthe member is calculated from the change in bending wave vibration inthe member created by the contact.

By bending wave vibration it is meant an excitation, for example by thecontact, which imparts some out of plane displacement to the member.Many materials bend, some with pure bending with a perfect square rootdispersion relation and some with a mixture of pure and shear bending.The dispersion relation describes the dependence of the in-planevelocity of the waves on the frequency of the waves.

Bending waves provide advantages, such as increased robustness andreduced sensitivity to surface scratches, etc. However, bending wavesare dispersive i.e. the bending wave velocity, and hence the “time offlight”, is dependent on frequency. In general, an impulse contains abroad range of component frequencies and thus if the impulse travels ashort distance, high frequency components will arrive first. U.S. patentapplication Ser. No. 09/746,405, International Publication Number WO01/48684 and International Application PCT/GB2002/003073, a correctionto convert the measured bending wave signal to a propagation signal froma non-dispersive wave source may be applied so that techniques used inthe fields of radar and sonar may be applied to detect the location ofthe contact.

A mounting 22 made of foamed plastics is attached to the underside ofand extends substantially around the periphery of the plate 12. Themounting 22 has adhesive surfaces whereby the member may be securelyattached to any surface. The mechanical impedance of the mounting andplate are selected so as to minimise reflections of bending waves fromthe plate edges.

The relationship between mechanical impedance of the mounting and theplate may be approximated by considering the one dimensional model shownin FIG. 3. The model comprises a waveguide 34 in the form of a beamwhich terminates at an edge mounting 36 having a termination impedance.An incident wave 38 travelling down the waveguide 34 is reflected by themounting 36 to form a reflected wave 40. The incident and reflectedwaves are plane waves travelling in the direction perpendicular to theedge. Assuming the mounting 36 satisfies the following boundaryconditions:

-   (i) the termination impedance only couples into the lateral    velocity, i.e. it does not provide any torque resistance; whereby    the bending moment is equal to zero at the edge and-   (ii) the ratio of the lateral shear force and the velocity at the    edge is equal to the terminal impedance;    -   the reflection coefficient at the mounting is given by:        ${R(\omega)} = \frac{{{- Z_{T}}/{Z_{B}(\omega)}} - i}{{Z_{T}/{Z_{B}(\omega)}} + 1}$        where Z_(T) is the termination impedance of the mounting and        Z_(B) is the mechanical impedance of the end of the waveguide,        given by        ${Z_{B}(\omega)} = {\frac{B\quad{k^{3}(\omega)}}{2\omega}\left( {1 + i} \right)}$    -   where k(ω), is the wavevector which may be expressed in terms of        the bending stiffness, B, and mass per unit area, μ, of the        panel, $k = {\left( \frac{\mu}{B} \right)^{1/4}\sqrt{\omega}}$

Thus, the reflection coefficient is determined by the ratio of theimpedances at the end of the waveguide and the mounting. Furthermore,the impedance of the waveguide is proportional to the square root offrequency and is both real and reactive in equal weights (i.e. π/4 phaseangle). Accordingly, the reflection coefficient is likely to be stronglyfrequency dependent.

The reflection coefficient vanishes, i.e. bending wave energy isstrongly absorbed in a frequency band around ω₀, if the followingcondition is satisfied:Z _(T) =−iZ _(B)(ω₀)

Thus, the termination impedance of the mounting must have both real andimaginary components, or, equivalently, the mounting should be bothresistive and compliant.

The plate may be, for example, 1 mm thick polycarbonate sheet which hasmass per unit area, μ=1.196 kg m⁻² and bending stiffness, B=0.38 N m.The equations above can be used to calculate the impedances of the plateand absorber required to strongly absorb bending wave energy around thechosen angular frequency ω₀=2π(900 Hz).

The impedance, per unit width for a 1 mm beam approximation of the plateisZ _(B)(ω₀)=(1+i)33.8 N s m⁻².

The properties of the absorber which provide the desired absorption arethus: Resistance per unit width,Re(Z _(T))=Im[Z _(B)(ω₀)]=33.8 N s m².Stiffness per unit width,−iIm(Z _(T))ω₀ =Re[Z _(B)(ω₀)]ω₀=1.91×10⁵ N m⁻².

The reflection coefficient is a unitless complex number. FIGS. 4 a and 4b are graphs showing the amplitude and phase of the reflectioncoefficient R(ω) varying with frequency. The amplitude of the reflectioncoefficient is zero and its phase is reversed for ω₀ approximately equalto 900 Hz.

In FIGS. 5 a and 5 b, the plate 12 has uniform surface roughness in theform of a raised surface pattern 28,29. The stylus 18 is drawn acrossthe surface along a path 30 and as it crosses a raised part or line ofthe pattern it generates bending waves 32 in the member. Thus contactfrom the stylus 18 provides a source of bending wave vibration in themember. In FIG. 5 a, the surface pattern 28 is a periodic pattern ofraised crossed lines and in FIG. 5 b, the surface pattern 29 is a randomrelief pattern.

In the embodiments of FIGS. 2, 5 a and 5 b, as the contact moves overthe rough surface of the member, bending waves radiate isotropically inthe member from the point of contact. The displacement of the member ata distance, x, from the point of contact is related to the displacementat the point of contact by a transfer function, H(ω; x). At distanceslarger than the wavelength, λ=2π/k(ω), the transfer function can beapproximated as,${{H\left( {\omega;x} \right)} = {\frac{A}{\sqrt{k(\omega)}x}{\mathbb{e}}^{{\mathbb{i}}\quad{k{(\omega)}}x}}},$where A is a constant and k(ω), is the wavevector defined previously.Although H(ω; x) strictly only applies to bending waves on an infiniteplate, since the mounting strongly absorbs bending wave vibrations, therelationship is satisfied. The transfer function shows that where asource of bending waves emits a purely sinusoidal frequency with angularfrequency, ω₀, the phase difference Δθ₁₂ between displacements at twolocations which are at distances, x₁ and x₂, from the point of contactfor the source is:exp(iΔθ ₁₂)=exp[ik(ω₀)(x ₁ −x ₂)]This implies the following relationship between the phase angledifference, the path length difference Δx=(x₁−x₂) and an integer n₁₂.Δθ₁₂=θ₁−θ₂ =k(ω₀)Δx ₁₂+2πn ₁₂

FIG. 6 shows the steps in the method for using this equation todetermine the contact location:

-   (a) Measure a bending wave signal with each sensor to give measured    bending wave signals W_(i)(t) and W_(j)(t),-   (b) Calculate the phase angles θ_(i)(t) and θ_(j)(t) of the measured    bending wave signals W_(i)(t) and W_(j)(t),-   (c) Calculate the difference between the two phase angles θ_(i)(t)    and θ_(j)(t)-   (d) Calculate the location of the contact from    k(ω₀)Δx _(ij)=Δθ_(ij)−2πn _(ij)

FIG. 7 a shows a schematic block diagram of a device for calculating thephase angle θ_(j) of a bending wave signal W_(j)(t) measured by one ofthe sensors. The signal W_(j)(t) is a random signal and is thusuncorrelated over long time scales. The signal is first amplified by anamplifier 42 and then processed by an analogue band-pass filter 44 witha pass-band centered at ω₀ and a bandwidth of Δω.

A moving source of bending waves may demonstrate the Doppler effect,whereby a bending wave which has frequency ω₀ and is emitted by a sourcemoving at velocity ν towards a point on a member arrives at that pointwith a different frequency defined by ω₀ −k(ω₀)ν. The maximum angularfrequency shift between bending waves at two different points on themember is therefore 2k(ω₀)ν_(max), where ν_(max) is the maximum velocityof the moving source. If the angular frequency shift becomes larger thanthe width of the band pass filter, the phase difference equation abovedoes not hold. Accordingly, the bandwidth Δω of the filter 44 is set tobe greater than this maximum frequency shift and thus obeys therelationship:Δω>>2k(ω₀)ν_(max)

After processing by the filter 44, the resulting filtered signalW′_(j)(t) is an amplitude and phase modulated carrier with frequency ω₀and is defined by:W′ _(j)(t)=A _(i)(t)sin └ω₀ t+θ _(j)(t)┘where A_(j)(t) and θ_(j)(t) are the amplitude and phase of the signal.Both fluctuate over a timescale Δt determined by the bandwidth of thefilter, namely Δt=2π/Δω. The maximum frequency at which independentphase angle measurements may be taken from the output of the bandpassfilter is $\frac{1}{\Delta\quad t}.$Since a touch sensor typically provides an updated measurement of thecontact position every 10 ms, the condition for the minimum frequency ofpositional measurement is Δt<10 ms.

The filtered signal W′_(j)(t) is then passed simultaneously to twoanalogue phase detectors 46. Such detectors are well known in the art,for example, see p644 of “The Art of Electronics” by Horowitz and Hill.Reference signals each having frequency ω₀ but a phase difference of π/2are also fed to the two phase detectors. The outputs of the phasedetectors are passed through low-pass filters 48 each having frequencycut-offs of approximately Δω/2. The outputs of the low-pass filters areproportional to cos(θ_(j)) and sin(θ_(j)) respectively. These outputsare then digitized by digitizers 50 and processed by processor 52 togive the phase angle θ_(j).

FIG. 7 b shows how the reference signals used in FIG. 7 a may begenerated. A second bending wave signal W_(i)(t) is measured at a secondsensor. The signal is fed through an amplifier 42 and analogue band-passfilter 44 to generate a filtered signal W′_(i)(t). The filtered signalW′_(i)(t) forms the reference signal which is fed directly to one phasedetector 46. The filtered signal is also fed to the second phasedetector 46 via a device which shifts its phase by π/2. The phaseshifted signal is used as the reference signal to the second phasedetector 46.

FIGS. 8 a to 8 d show how the phase angle differences and hence the pathlength differences may be used to calculate the location of the contact.The equation in step (d) of FIG. 6 defines a hyperbolic curve which canbe overlaid on the plate 12. FIG. 8 a shows the three hyperbolic curves26 which are generated using three different values of n_(lm) and thecalculated phase angle difference for a pair of sensors 16 mounted oneon each end of the short sides of the plate 12. Similarly FIGS. 8 b and8 c show the hyperbolic curves 26 which are generated by the phase angledifference and different values of n_(lm) for two other pairs ofsensors. FIG. 8 d shows all the hyperbolic curves created by thesensors. The contact location 24 is the point of intersection of threehyperbolic curves, one from each pair of sensors. From the contactlocation 24, the correct value of n_(lm) may be inferred.

A method of inferring n is implemented using the embodiment shown inFIG. 9. The bending wave signal W₁(t) measured by each sensor issimultaneously processed by two band-pass filters 48,54. Two phaseangles, one for each filter, are calculated, for example as described inFIG. 7. The filters 48, 54 have slightly different pass-band frequencieswhereby two phase angle differences, one for each pass-band frequency,are provided by each pair of sensors.

The phase angle differences Δθ_(a), Δθ_(b) from the sensors may bedefined asΔθ_(a) =k(ω₀+ω_(δ))Δx+2πn _(a)Δθ_(b) =k(ω₀−ω_(δ))Δx+2πn _(b)where Δx is a single path-length difference defined by the contact andthe position of the sensors.

The correct combination (n_(a), n_(b)) may be determined as thecombination of values that minimise the expression:${\frac{{\Delta\quad\theta_{a}} - {2\pi\quad n_{a}}}{k\left( {\omega_{0} + \omega_{\delta}} \right)} - \frac{{\Delta\quad\theta_{b}} - {2\pi\quad n_{b}}}{k\left( {\omega_{0} - \omega_{\delta}} \right)}}$The path length difference may then be estimated as:${\Delta\quad x} = {\frac{1}{2}\left( {\frac{{\Delta\quad\theta_{a}} - {2\pi\quad n_{a}}}{k\left( {\omega_{0} + \omega_{\delta}} \right)} + \frac{{\Delta\quad\theta_{b}} - {2\pi\quad n_{b}}}{k\left( {\omega_{0} - \omega_{\delta}} \right)}} \right)}$

Another pair of sensors may then be used to determine a second pathlength difference. Each path length difference defines a hyperboliccurve on the panel. The intersection point of these two hyperboliccurves is the location of the contact.

Note that hyperbolae are defined by values of path length difference orΔx. In general, for a given phase-angle difference, several values of Δxare possible (corresponding to different values of n). The advantage ofusing two frequencies is that a single value of Δx can be obtained foreach pair of sensors (using the method of minimizing the expressiondescribed above). The determination of the exact value of Δx, ratherthan a series of possible values, constrains the location of the contactto a single hyperbola, rather than a series of hyperbolae. The locationcan be determined exactly from the intersection of two hyperbolae, andhence from two pairs of sensors.

FIG. 10 shows an alternative method for calculating the location of thecontact from the equation above, namely

-   i. Measure a pair of bending wave signals W_(i)(t) and W_(j)(t), one    signal being measured by a sensor;-   ii. Calculate the dispersion corrected correlation function of the    two signals using the method described in FIGS. 11 and 11 a;-   iii. Calculate the initial position of the contact using the    dispersion corrected correlation function, as described in FIGS. 11    and 11 a;-   iv. Remeasure bending wave signals W_(i)(t) and W_(j)(t);-   v. Calculate the phase angle of each signal—for example as described    in FIGS. 7 a and 7 b;-   vi. Calculate the difference between the phase angles;-   vii. Select the value of n_(lm) which minimizes the change in the    path length difference;-   viii. Plot the hyperbola defined by    k(w ₀)Δx _(ij)=Δθ_(ij)−2πn _(ij)-   ix. Repeat steps (iv) to (viii), remeasuring the bending wave    signals at regular intervals Δt, for example Δt=2π/Δω.

At step (viii), a minimum of two hyperbolae from different pairs ofsensors are required to determine the position of the contact. Thereforethe entire is performed simultaneously for at least two pairs ofsensors.

FIG. 11 shows a method of calculating the dispersion correctedcorrelation function to reveal the difference in path length between thecontact location and the sensors. The method set out below summarizesthe information in PCT/GB2002/003073. The method comprises the followingsteps:

-   (a) Measure two bending wave signals W₁(t) and W₂(t);-   (b) Calculate the Fourier transform of W₁(t) and W₂(t) to arrive at    Ŵ₁(ω) and Ŵ₂ (ω) and hence the intermediate function Ŵ₁(ω)Ŵ₂*(ω);    where Ŵ₂*(ω) is the complex conjugate Fourier transform, t    represents time ω is 2πf where f is frequency.-   (c) Calculate a second intermediate function M(ω) which is a    function of Ŵ₁ (ω)Ŵ₂*(ω)-   (d) and (e) at the same time as performing steps (a) to (c), the    frequency stretching operation f(ω)=ν(μ/B)^(1/4)√{square root over    (ω)} is calculated using the predetermined panel dispersion relation    k=(μ/B)^(1/4)√{square root over (ω)}.-   (f) M(ω) and f(ω)=ν(μ/B)^(1/4)√{square root over (ω)} are combined    to arrive at the dispersion corrected correlation function:    ${{G(t)} = {\frac{1}{2\pi}{\int_{- \infty}^{+ \infty}{{M\left\lbrack {f(\omega)} \right\rbrack}{\exp\left( {{\mathbb{i}}\quad\omega\quad t} \right)}\quad{\mathbb{d}\omega}}}}};{and}$-   (g) the dispersion corrected correlation function is plotted against    time with a peak occurring at time t₁₂ as shown in FIG. 11 a;-   (h) Δx₁₂ is calculated from t₁₂; Δx₁₂ is the path-length difference    between the path lengths x₁ and x₂ from the first and second sensors    to the contact.-   (i) Δx₁₂ defines a hyperbolae which may be plotted as in FIG. 7 to    calculate the location of the contact.

As with the method of FIG. 10, a minimum of two hyperbolae are requiredto determine the location of the contact. Thus the ways of generatingmore hyperbolae discussed above apply equally to this method.

The second intermediate function M(ω) may simply be Ŵ₁(ω)Ŵ₂*(ω) whichgives a standard dispersion corrected correlation function.Alternatively, M(ω) may be selected from the following functions whichall yield phase equivalent functions to the standard dispersioncorrected correlation function:$\begin{matrix}{{\text{(a)}\quad{M(\omega)}} = \frac{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}{{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}}} \\{{\text{(b)}\quad{M(\omega)}} = \frac{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}{\sqrt{{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}}}}\end{matrix}$

-   -   (c) M(ω)=Ŵ₁(ω)Ŵ₂*(ω)φ└|Ŵ₁(ω)Ŵ₂*(ω)|┘ where φ(x) is real valued        function

(d) M(ω)=Ŵ₁(ω)Ŵ₂*(ω)ψ(ω) where ψ(ω) is a real valued function

Alternatively, M(ω) may be the function {circumflex over (D)}(ω) whichis the Fourier transformation of the correlation function D(t):

 D(t)=∫_(−∞) ^(∞) W ₁(t+t′)W ₂(t′)dt′

The steps are calculate D(t); calculate {circumflex over (D)}(ω) andapply a frequency stretching operation to arrive at the dispersioncorrected correlation function:${G(t)} = {\frac{1}{2\pi}{\int_{- \infty}^{+ \infty}{{\hat{D}\left\lbrack {f(\omega)} \right\rbrack}{\exp\left( {{\mathbb{i}}\quad\omega\quad t} \right)}\quad{{\mathbb{d}\omega}.}}}}$

Alternatively, at step (f) the following dispersion correctedcorrelation function may be calculated:${G(t)} = {\frac{1}{2\pi}{\int_{- \infty}^{+ \infty}{{{\hat{W}}_{1}\left\lbrack {f(\omega)} \right\rbrack}{{\hat{W}}_{2}^{*}\left\lbrack {f(\omega)} \right\rbrack}{\phi_{12}\left\lbrack {f(\omega)} \right\rbrack}{\exp\left( {{\mathbb{i}}\quad\omega\quad t} \right)}\quad{\mathbb{d}\omega}}}}$where${\phi_{12}^{*}(\omega)} = {{\sum\limits_{j}{{{\hat{W}}_{1,j}(\omega)}{{\hat{W}}_{2,j}^{*}(\omega)}{\exp\left\lbrack {{- {\mathbb{i}}}\quad{k(\omega)}\Delta\quad x_{j}} \right\rbrack}}}}$where {Ŵ_(1,j)(ω)} and {Ŵ*_(2,j)(ω)} are the Fourier transformation andcomplex conjugate Fourier transformation of two measured bending wavesignals {W_(1,j)(t)} and {W_(2,j)(t)} and {Δx_(j)} is the path-lengthdifference.

A sensor may act as both the first and second sensor whereby thedispersion corrected correlation function is an autocorrelationfunction. The autocorrelation function may be calculated applying thesame steps for the dispersion corrected correlation function usingW₁(t)=W₂(t).

FIG. 12 a shows a contact sensitive device which also operates as aloudspeaker. FIG. 12 b shows a method for partitioning the audio signaland measured signal into two distinct frequency bands so that thecontribution of the audio signal to the processed measured signal issuppressed. The device comprises a member 106 in which bending waves aregenerated by an emitting transducer or actuator 108 and the contact. Theemitting transducer applies an audio signal to the member 106 togenerate an acoustic output. Before being applied to the member, theaudio signal is filtered by a low pass filter 112 which, as shown inFIG. 12 b, removes the audio signal above a threshold frequency f₀.

As shown in FIG. 12 b, the contact generates a signal which has a poweroutput which is substantially constant over a large frequency band. Thesignal from the contact and the audio signal sum to give a combinedsignal which is passed through a high pass filter 114 to remove thesignal above the threshold frequency f₀. The filtered signal is thenpassed to a digitizer 116 and onto a processor 118.

1. A contact sensitive device comprising: a member capable of supportingbending waves; a plurality of sensors mounted on the member formeasuring bending wave vibration in the member, wherein each of thesensors determines a measured bending wave signal; and a processorresponsive to the measured bending wave signals to calculate a locationof a contact on the member; the processor calculating a phase angle foreach measured bending wave signal and a phase difference between thephase angles of least two pairs of sensors so that at least two phasedifferences are calculated from which the location of the contact isdetermined.
 2. A contact sensitive device according to claim 1comprising an absorber at edges of the member whereby reflected wavesare suppressed.
 3. A contact sensitive device according to claim 2,wherein mechanical impedance of the absorber and the member are selectedso as to reduce reflections of bending waves from the edges of themember.
 4. A contact sensitive device according to claim 3, wherein theimpedances are selected so that bending wave energy is strongly absorbedin a frequency band around a chosen frequency ω₀.
 5. A contact sensitivedevice according to claim 4, wherein the impedances are selected tosatisfy the following equation:Z _(T) =−iZ _(B)(ω₀) where Z_(T) is the termination impedance of theabsorber and Z_(B) is the mechanical impedance of the edge of themember.
 6. A contact sensitive device according to claim 4, comprising aband-pass filter for filtering each measured bending wave signal, thefilter having a pass-band centered at the chosen frequency ω₀ and abandwidth of Δω.
 7. A contact sensitive device according to claim 6,wherein the bandwidth Δω of the filter obeys the relationship:Δω>>2k(ω₀)ν_(max) where ν_(max) is the maximum lateral velocity of thecontact.
 8. A contact sensitive device according to claim 2 wherein theabsorber is made from foamed plastics.
 9. A contact sensitive deviceaccording to claim 1, wherein the member comprises a raised pattern onits surface whereby a contact drawn across the surface provides a forceto the member to generate bending waves in the member.
 10. A contactsensitive device according to claim 9, wherein the pattern is randomwhereby a contact traveling over the surface of the member generates arandom bending wave signal.
 11. A contact sensitive device according toclaim 10, wherein the pattern is formed from an anti-reflective coating,an anti-glare surface finish, or an etched finish.
 12. A contactsensitive device according to claim 1, further comprising at least twoband-pass filters which have different pass-band frequencies and whichsimultaneously process the bending wave signals measured by a pair ofsensors whereby a phase angle difference for each pass-band frequency isprovided by the pair of sensors.
 13. A contact sensitive deviceaccording to claim 1, comprising four sensors on the member.
 14. Acontact sensitive device according to claim 1 comprising three or moresensors on the member.
 15. A contact sensitive device according to claim1, comprising means for determining an initial location of the contactusing a dispersion corrected correlation function of pairs of measuredbending wave signals and means for determining subsequent locations ofthe contact using the phase angle difference between pairs of measuredbending wave signals.
 16. A contact sensitive device according to claim1, wherein an initial location of the contact is determined using adispersion corrected correlation function of pairs of measured bendingwave signals and wherein subsequent locations of the contact aredetermined using a phase angle difference between pairs of measuredbending wave signals.
 17. A contact sensitive device according to claim1, further comprising a phase detector for determining the phase angle.18. A contact sensitive device according to claim 17, wherein theprocessor comprises a low-pass filter and a digitizer for determiningthe phase angles.
 19. A contact sensitive device according to claim 1,wherein the member is an acoustic radiator and an emitting transducer ismounted to the member to excite bending wave vibration in the member togenerate an acoustic output.
 20. A contact sensitive device according toclaim 19, comprising means for ensuring that the acoustic output andmeasured bending wave signals are in discrete frequency bands.
 21. Acontact sensitive device according to claim 19, further comprising oneor more filters to separate the acoustic output from the measuredbending wave signals.
 22. A contact sensitive device according to claim1, further comprising an emitting transducer mounted to the member toexcite bending wave vibration in the member.
 23. A contact sensitivedevice according to claim 22, wherein the emitting transducer alsofunctions as one of the sensors.
 24. A contact sensitive deviceaccording to claim 1 wherein the member is transparent.
 25. A contactsensitive device according to claim 1 wherein the member is a liquidcrystal display screen comprising liquid crystals utilized to excite orsense bending wave vibration in the member.
 26. A method of determininginformation relating to a contact on a contact sensitive devicecomprising: providing a member capable of supporting bending waves and aplurality of sensors mounted on the member for measuring bending wavevibration in the member; applying a contact to the member at a location,using each of the sensors to determine a measured bending wave signal;and calculating the location of a contact from the measured bending wavesignals by calculating a phase angle for each measured bending wavesignal, calculating a phase difference between the phase angles of atleast two pairs of sensors and determining the location of the contactfrom the at least two calculated phase differences.
 27. A methodaccording to claim 26, wherein the plurality of sensors is three ormore.
 28. A method according to claim 26, comprising suppressingreflected waves by placing an absorber at the edges of the member.
 29. Amethod according to claim 28, comprising selecting the mechanicalimpedances of the absorber and the member so as to reduce reflections ofbending waves from the edges of the member.
 30. A method according toclaim 29, comprising selecting the impedances so that bending waveenergy is strongly absorbed in a frequency band around a chosenfrequency ω₀.
 31. A method according to claim 30, comprising selectingthe impedances to satisfy the following equationZ _(T) =−iZ _(B)(ω₀) where Z_(T) is the impedance of the absorber andZ_(B) is the impedance of the edge of the member.
 32. A method accordingto claim 30, comprising filtering each measured bending wave signal by aband-pass filter having a pass-band centered at the chosen frequency ω₀and a bandwidth of Δ_(ω.)
 33. A method according to claim 26, comprisingapplying the phase difference equation:Δθ_(lm)=θ_(l)−θ_(m) =k(ω₀)Δx _(lm)+2πn _(lm) to determine the locationof the contact, where θ_(i) is the phase angle of a measured bendingwave signal, x_(i) is the distance from the contact location to eachsensor, Δx_(lm)=x_(l)−x_(m) is the path length difference of twosensors, k(ω) is the wavevector and n_(lm) is an unknown integer.
 34. Amethod according to claim 33, comprising selecting the member toconstrain the magnitude of Δx_(lm) to values less than one half of awavelength so that n_(lm) is determined from |Δθ_(ml)−2πn_(lm)|<π.
 35. Amethod according to claim 33, comprising determining an initial locationof the contact using the dispersion corrected correlation function of apair of measured bending wave signals and selecting a value of n_(lm)which minimizes change in the path length difference.
 36. A methodaccording to claim 33, comprising selecting a series of values ofn_(lm), combining the series of values with each phase angle differenceto define a series of path length differences, plotting the series ofgraphs of the path length differences, and inferring the true value ofn_(lm) from a point at which a large number of the graphs intersect. 37.A method according to claim 26, comprising calculating multiple phaseangle differences from the pairs of phase angles, plotting a graph ofeach path length difference and selecting a point at which a largenumber of the hyperbolae intersect to be the location of the contact.38. A method according to claim 26, comprising dividing the measuredbending wave signals from each sensor into at least two discretefrequency bands and calculating a phase angle difference for a pair ofsensors for each frequency band.
 39. A contact sensitive devicecomprising: a member capable of supporting bending waves; a plurality ofsensors mounted on the member for measuring bending wave vibration inthe member; and means for calculating the location of a contact from themeasured bending wave signal by calculating a phase angle for eachmeasured bending wave signal, calculating a phase difference between thephase angles of at least two pairs of sensors and determining thelocation of the contact from the at least two calculated phasedifferences.